Dennis,
     
        If the base laminate is made of a high Tg laminate then the
     "Z" expansion (PPM) is less likely to exhibit the type of failures
     as you mentioned. High Tg laminates are more favorable for products 
     that have multiple thermal excursions. (i.e., two or three assembly 
     thermal excursions like wave, IR, FTIR, Hot Bar or forced air) Another 
     thing that that is critical is the elongation and tensile strength of 
     the plated copper in the hole. Of course ample (correct) copper 
     plating thickness is essential. High Aspect Ratio plating has a 
     tendency to produce a "dog boning" appearance that could have a less 
     than desired thickness in the center and too much at the knees. These 
     types of deposits each will have polar extremes as far as E and T 
     results. A good (level) and evenly plated (distributed) PTH will have 
     more consistent results and can easily then have variable copper 
     plated thru hole thickness' tested to find what thickness will produce 
     none of what you mentioned. 
     You can pre-bake parts to remove and potential moisture that could 
     increase your "Z" expansion prior to thermal activity.
     Some tests have indicated that a PTH >1.5 mils reduces PTH anomalies
     but at >2.0 mils they come back. Other old timers swear that a panel
     pattern copper plate combo will help with this kind of problem.
     (??????I dunno...never tried it!)
     
     Good luck.
     
     DHH


______________________________ Reply Separator _________________________________
Subject: High Z Axis Expansion Board Materials
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    12/2/95 5:27 PM


We receive questions about plating high Z axis expansion board materials.
 When the expansion from assembly soldering is too great, surface pads around
the holes lift, or the plated barrel cracks.  Does anyone have a standard 
practice to prevent these phenomena when using laminate with great amounts of
 Z axis expansion (we think more than 6%)?
     
Thicker plating in the hole?
Alternate metals plated?
     
Dennis Fritz
MacDermid, Inc.
245 Freight Street
Waterbury CT 06702
Phone 203-575-5740